1. Field of the Invention
The present invention relates to a liquid crystal display device and, more specifically, to a driving technique for a liquid crystal panel of a liquid crystal display device that employs common inversion driving.
2. Description of Related Art
In driving of the liquid crystal display, in order to avoid so-called ghosting, the inversion drive is performed. In the inversion drive, the polarity of a driving voltage applied to each pixel (that is, potential polarity of a pixel electrode for a counter electrode) at an appropriate time interval. As an example of inversion drive, in a frame inversion drive, a driving voltage of each pixel is inverted for every one frame period.
However, in a simple frame inversion drive, flickers tend to become apparent. Therefore, when performing the frame inversion drive, a polarity of the driving voltage applied to each pixel is inverted at adequate spatial interval for suppressing the flickers. For example, one of widely-employed inversion drive techniques is the dot inversion drive which drives pixels in such a manner that the polarities of the driving voltages for neighboring pixels become opposite from each other both in a vertical direction and a horizontal direction. Another one of those widely-employed inversion drive techniques is the horizontal line inversion drive which inverts a polarity of the driving voltage for each pixel by every prescribed number of horizontal line(s). The inversion cycle of the horizontal lines for inverting the driving voltage can be determined variously. For example, the horizontal line inversion drive which inverts a polarity of the driving voltage for every horizontal line is referred to as the 1H inversion drive. The horizontal line inversion drive which inverts the polarity of the driving voltage by a unit of two horizontal lines may be referred to as the 2H inversion drive.
The inversion drive can be classified from another viewpoint based on a method for driving the counter electrode. That is, the inversion drive can be largely classified into the common constant drive and the common inversion drive. The common constant drive is a driving method which keeps potential of the counter electrode constant. The common inversion drive is a driving method which inverts the potential of the counter electrode in accordance with a cycle at which a polarity of the driving voltage of the pixel is inverted. The common inversion drive is preferable than the common constant drive if it can be employed, since it is capable of reducing an operating voltage of a driving circuit which generates the driving voltage of the pixel. When the dot inversion drive is employed, the common inversion drive cannot be employed. Thus, the common constant drive is employed for such case. However, in a case where the horizontal line inversion drive is to be performed, the common inversion drive is normally employed.
One of the problems for employing the common inversion drive is that it requires large power for driving the counter electrode, since parasitic capacitance of the counter electrode is generally large. This is not preferable, because it increases the power consumption of the liquid crystal display device.
One of the methods for reducing the power consumption of the liquid crystal display device in a case that the common inversion drive is employed is to short-circuit source lines (also referred to as data lines or signal lines in general) of the liquid crystal display panel and the common electrodes before driving the common electrodes. This makes it possible to utilize electric charges accumulated in the source lines and the counter electrode effectively and to reduce the power required for driving the source lines and the counter electrode effectively. Such technique is disclosed in Japanese Laid-Open Patent Application JP-P2007-101570A (referred to as Patent Document 1 in the following), for example.
FIG. 1 is a block diagram showing a structure of a liquid crystal display device disclosed in the Patent Document 1. A driving device 600 for driving a liquid crystal display panel 512 includes a source line driving circuit 520 for driving source lines S1 to Sn and a power supply circuit 542. The power supply circuit 542 includes a counter electrode voltage supply circuit 560 which generates a counter electrode voltage to be supplied to a counter electrode VCOM, and supplies the counter electrode voltage to the counter electrode VCOM. The source line driving circuit 520 includes short-circuiting circuits SHT1 to SHTN for short-circuiting the counter electrode VCOM and the source lines S1 to Sn. The short-circuiting circuits SHT1 to SHTN operate in response to a polarity signal POL and a control signal BSC generated in accordance with an electric charge reuse period designating signal. The power supply circuit 542 includes the counter electrode voltage supply circuit 560 which generates the driving voltage of the counter electrode VCOM in accordance with the polarity of the driving voltage of the pixel, and a voltage setting circuit 562 which supplies either the voltage supplied from the counter electrode voltage supply circuit 560 or a set voltage VSET to the counter electrode VCOM. The set voltage VSET is a potential close to a ground potential VSS. The voltage setting circuit 562 operates in response to the control signal VSC that is generated in accordance with the polarity signal POL and the electric charge reuse period designating signal.
FIG. 2 is a timing chart showing an operation of the liquid crystal display device shown in FIG. 1. In FIG. 2, a curve with reference code S1 shows variation of a potential of a given source line Sj, and a curve with reference code VCOM shows variation of a potential of the counter electrode VCOM. Note that FIG. 2 shows an operations of the liquid crystal display device when the liquid crystal display panel 512 is “normally-white”.
In the liquid crystal display device shown in FIG. 1, driving procedures for the source lines S1 to Sn and the counter electrode VCOM are different for a case where the polarity of the driving voltage of the pixel is changed from positive to negative and for a case where the polarity is changed from negative to positive. In other words, the driving procedures are different for a case where the counter electrode VCOM is pulled up to a potential VCOMH and for a case where it is pulled down to a potential VCOML. Note here that the potential VCOMH is a predetermined positive potential that is to be set for the counter electrode VCOM when the polarity of the driving voltage of the pixel is negative, and the potential VCOML is a predetermined negative potential that is to be set for the counter electrode VCOM when the polarity of the driving voltage of the pixel is positive.
When the polarity of the driving voltage of the pixel is changed from positive to negative, first, the counter electrode VCOM is driven to the setting potential VSET. Specifically, the voltage setting period signal is asserted, the setting potential VSET is selected by the voltage setting circuit 562, and the counter electrode VCOM is driven to the setting potential VSET. Subsequently, the electric charge reuse period designating signal is asserted. Thereby, the counter electrode VCOM and the source lines S1 to Sn are short-circuited through the short-circuiting circuits STH1 to STHn. With this, the counter electrode VCOM and the source lines S1 to Sn come to have a mean potential of the source lines S1 to S2 and the counter electrode VCOM without electric power consumption. In this procedure, the counter electrode VCOM is driven to the setting potential in advance.
This is done to prevent the source lines S1 to S2 from having a negative potential, when the counter electrode VCOM and the source lines S1 to Sn are short-circuited. After the counter electrode VCOM and the source lines S1 to Sn are short-circuited, each pixel connected to the source lines S1 to Sn is driven to a predetermined driving voltage.
In the meantime, when the polarity of the driving voltage of the pixel is changed from negative to positive, the counter electrode VCOM and the source lines S1 to Sn are short-circuited (without driving the counter electrode VCOM to the setting potential VSET). After the counter electrode VCOM and the source lines S1 to Sn are short-circuited, each pixel connected to the source lines S1 to Sn is driven to a predetermined driving voltage.
In any cases, by short-circuiting the counter electrode VCOM and the source lines S1 to Sn, the electric charges accumulated in the counter electrode VCOM or the source lines S1 to Sn are reutilized effectively. As a result, the power required for driving the counter electrode VCOM and the source lines S1 to Sn can be reduced.